Fuel cell power systems which utilize air as an oxygen source such as disclosed in U.S. Pat. No. 3,576,677 to Keating, Jr., et al. will typically supply the air with a constant air supply blower having a modulated outlet control valve to maintain optimum oxygen utilization in the cells, thereby permitting water recovery and good cell performance. The modulated valve will generally be a relatively slow motorized valve. For the vast majority of operating conditions, such an oxygen supply system is perfectly adequate. An exception can occur, however, when increased power load demands are imposed on the cells. One of the positive aspects of fuel cell power plant systems is that they are substantially instantaneous in responding to demands in increased current output or load. When an increase in load is met by a fuel cell power plant, a concurrent and equally quick increase in reactant supply should also occur to ensure proper operation of the fuel cell power plant. This is especially true for large scale increases in power output. This relatively instantaneous increase in oxygen supply will not occur with the prior art motorized modulated air supply valve because this type of valve is incapable of such quick changes in its capacity, especially in large size butterfly valves. It can take the conventional motorized modulating valve a number of seconds to adjust its feed rate to a new higher rate required because of an increase in load imposed on the power plant. During this interval, oxygen starvation can occur causing unstable operating conditions. Reduced cell voltage, increased current, fuel starvation and anode corrosion can result. The power plant will fail to produce the power demanded and may shut down due to out of limits conditions.